1. Field of the Invention
The present invention relates to a film growing method, particularly to a method for growing aluminum indium nitride films.
2. Description of the Prior Art
Aluminum indium nitride (AlInN or InAlN) is an intrinsic n-type semiconductor. The energy band gap can be varied from 0.7 eV to 6.2 eV with its chemical composition. Since the energy gap covers a large range, it may be applied to high power and high frequency devices, light emitting diodes (LEDs), or full-spectrum solar cells. Regarding Al-rich AlInN, when the In ratio extends to 17-18%, there is no piezoelectric polarization but only spontaneous polarization because the lattice parameter of AlInN completely matches with the lattice parameter of GaN. Thus it is considered as a highly potential semiconductor material for developing high electron mobility transistor (HEMT) and metal-oxide-semiconductor field-effect transistor (MOSFET). Regarding In-rich AlInN, it is highly potential to be applied to solar cells due to its low energy gap and high light-absorbing rate. On the other hand, AlInN may be used with InN or InGaN to form multiple quantum wells (MQWs) structure to be applied to optoelectronic devices.
Although AlInN may be widely used, it is very difficult to be prepared. The difficult part of preparing ternary alloy formed by InN and AlN lies in that growth of AlN has to be performed under a high temperature ranging from 600° C. to 1200° C.; whereas growth of InN should be performed under a temperature lower than 600° C., otherwise InN will be easily to undergo thermal decomposition. Since the difference in preparing temperatures of each preparation is very large, it is difficult to form a ternary alloy simultaneously having both single phase and high quality under a single temperature condition. For example, in order to get AlN, a high temperature process is used when forming Al-rich AlInN, thus resulting in unstable InN and the composition of the ternary group-III nitrides may not be controlled precisely and stably. Therefore epitaxial quality of the film is affected.
Currently, substrates for forming group-III nitrides mainly are silicon substrates, silicon carbide substrates, and sapphire substrates. In which monocrystalline silicon substrates are advantageous in price, size and quality aspects. Many related industries and researchers have developed a few results. However, in order to improve the problem of epitaxial quality degradation caused by lattice mismatching between materials and thermal expansion differences, generally, a buffer layer should be formed on the substrate to grow a high quality nitride film before above three types of substrates being used. For example, before growing group-III nitride films, such as gallium nitride (GaN) or aluminum indium nitride (AlInN), AlN is usually used as a buffer layer. The process makes the preparing method complicated and with a high cost. Besides, AlN brings additional insulating problems to limit developments and popularity of related products.
If AlInN film can be grown without requirement of a buffer layer such that good lattice matching and thermal stability between the substrate and AlInN film are achieved, the usage of AlInN may largely increase.